In a typical cellular network, also referred to as a wireless communication system, User Equipment (UE), communicate via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A UE is referred to as a mobile terminal by which a subscriber can access services offered by an operator's CN. The UEs may be for example communication devices such as mobile telephones, cellular telephones, laptops, tablet computers or vehicle-mounted mobile devices, enabled to communicate voice and/or data. The wireless capability enables to communicate voice and/or data, via the RAN, with another entity, such as another UE or a server.
The cellular network covers a geographical area which is divided into cell based areas. Each cell area is served by a Base Station (BS), or Radio Base Station (RBS), which is also referred to as e.g. “evolved NodeB”, “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used.
The RBSs may be of different classes such as e.g. macro RBS, home RBS or pico RBS, based on transmission power and thereby also on cell size.
A cell is the geographical area where radio coverage is provided by the RBS at a RBS site. One RBS may serve one or more cells. Further, each RBS may support one or several communication technologies. The RBSs communicate over the air interface operating on radio frequencies with the UEs within coverage range of the RBSs.
The Universal Mobile Telecommunication System (UMTS) is a third-generation, 3G, mobile communication system, which evolved from the second-generation, 2G, Global System for Mobile communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (W-CDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a RAN using W-CDMA. The 3rd. Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN (and GSM) based radio access network technologies.
The Long Term Evolution (LTE) mobile communication system is defined as the fourth-generation mobile communication technology standard within the 3GPP as to improve the UMTS to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lower costs. The UTRAN, being the radio access network of UMTS is further developed into an Evolved UTRAN (E-UTRAN), also referred to as a mobile broadband network, indicated as the radio access network of an LTE system. In an E-UTRAN, a UE is wirelessly connected to a RBS, commonly referred to as evolved NodeB (eNodeB or eNB).
FIG. 1 illustrates a block diagram of an E-UTRAN with RAN 100 comprising a first RBS 110, denoted as RBS-A that serves UEs 150, 152, 154, 156, located within the RBS-A's geographical area of service, called a first cell 112, or RBS-A's coverage. RBS-A serves the connected UEs 150, 152, 154, 156 depicted by means of dashed connecting lines. FIG. 1 illustrates two RBSs as an example. In practice a RBS is surrounded by- and connected to multiple RBSs.
The RAN of FIG. 1 additionally shows a neighboring second RBS 120, denoted as RBS-B comprised by the RAN, which has a geographical area of service, call a second cell 122, or cell coverage. RBS-B, although having UEs 150, 152, 154 within its area of service 122, is not serving one of these depicted UEs.
Both RBSs 110, 120, are communicatively connected via an X2-link 136 to each other enabling signaling, and are as well communicatively connected via respective S1-links 116 and 126, to a CN 140, comprising an Internet Protocol (IP) based Evolved Packet System (EPS) enabled to provide services to the UEs. The CN in an E-UTRAN system comprises a Mobility Management Entity (MME) which is the main signaling node in the EPC. The MME is responsible for initiating paging and authentication of the UE.
Other access technologies like GSM might apply a CN 140, comprising Radio Network Controllers, RNCs, and or Radio Base-station Controllers, RBCs, enabled to control the RBSs, and is, among other things, in control of management of radio resources in cells for which the RNC/RBC is responsible. The RNC/RBC enables communication between the RBSs. In general non-LTE networks have no direct links like the X2-link 136 between RBSs.
A RAN 100, such as an E-UTRAN, is often deployed on multiple carrier frequencies. A carrier frequency is the center frequency used for the radio communication between the RBS and the UE. Carrier frequencies are usually organized in radio frequency bands, the carrier frequencies bandwidth typically ranging from 5 to 20 MHz depending on the allocation of the Radio Frequency (RF).
A RBS may provide a number of radio cells on each carrier frequency, overlaid, overlaying or overlapping with each other or sectorized and pointing in different directions from the RBS.
FIG. 2A is a block-diagram showing a RBS 200 having a coverage in the shape of substantial circles 210, 220, each comprising a different carrier frequency, showing an a further example of a partly overlap. FIG. 2A is an example how a RBS in a multi-layered E-UTRAN could be implemented.
FIG. 2B is a block-diagram showing a RBS 250 having a coverage in the shape of substantial sectors 260, 270, also call beams, each comprising a different carrier frequency, showing a still further example of a partly overlap.
The UEs 150, 152, 154 within the overlapping area of coverage 112, 122 may access the CN 140 via a cell on either one of the overlapping carrier frequencies. UEs roaming in the network are moving between neighboring cells in order to stay in contact with the network. UE mobility between cells of different carrier frequencies is known as inter-frequency (IEF) mobility.
The CN 140 (or RBSs 110, 120), are usually in control of the mobility of UEs that are in connected mode. The term connected mode is used to denote the state of UEs with an active connection to the network, such as a Radio Resource Control (RRC) state RRC_CONNECTED in E-UTRAN.
In FIG. 1 the connected mode is depicted by the dashed lines between the RBS-A and the UEs 150, 152, 154. The RBSs perform a handover or relocation of a UE in connected mode when the UE is moving between cells. An IEF handover moves the connection of the UE between the cells controlled by the RBSs of different carrier frequencies.
UEs which do not have an active connection to the network are in idle mode. One example of an idle mode state is RRC_IDLE state in E-UTRAN.
Different cells and different carrier frequencies may offer system capacity that varies within a wide range. The cell configuration, the presence of radio interference, time-dispersion effects and the distribution of UEs within the cell affecting so called near-far-relations, are examples of factors influencing the system capacity. As to improve overall system performance a RBS has a mechanism to detect whether a relocation of UEs, eligible to be relocated, to neighbor RBSs would be beneficial for the system. The RBS driven relocation mechanism is known as Load-balancing. The purpose of load balancing is to distribute and equalize the traffic load presented to the E-UTRAN between the overlapping cells in such a way that the traffic load presented to each cell matches the traffic handling capacity of each cell in relation to the traffic handling capacity of the alternative overlapping cells
The load balancing mechanism is performed by the RBSs applying handovers in order to handle the mobility of UEs in connected mode. In order to distribute and equalize the traffic load among the cells, the E-UTRAN is enabled to relocate a number of UEs in connected mode to neighboring cells as to perform load balancing.
Only UEs in the overlap are eligible for a handover. However in general the RBSs do not know whether a particular UE resides in the overlap, if no positioning means are applied.
An example of an IEF load balancing technology has a number of basic characteristics, presented as sequential steps:
1. Each RBS in the network is configured with a number of known RBS relations to neighbor cells within the network and in a multi-layer network, it includes cell relations to neighbor cells on other carrier frequencies.
2. The RBS may configure a UE, connected to the RBS, to perform IEF measurements on the other carrier frequencies. As a result, the UE may report neighbor cells next to where the UE is connected to, located on other carrier frequencies, which are transmitting radio signals, which the UE can receive with good signal strength and radio link quality. Only UEs in the overlap are in fact eligible for a handover. However in general the RBSs do not know whether a particular UE resides in the overlap, when no positioning means are applied. The connected UEs configured by the RBS to perform IEF measurements on other frequencies are in general selected randomly, such that it can be expected that statistically a part of configured UEs are resident in the overlap.
3. The RBS may consider the IEF neighbor cells frequently occurring in such measurement reports as suitable target cells for load balancing versus the RBS where the UE is connected to. When suitable load balancing target cells are identified, the RBSs within the network may setup intra-network signaling relations between those RBS, wherein, for instance, traffic load and cell traffic capacity information are repeatedly exchanged.
4. Based on the exchange of traffic load and cell traffic capacity information, the RBS is able to identify target cells on the other carrier frequencies to which there is a significant traffic load imbalance, hence where a certain amount of traffic load should be transferred to, in order to mitigate the present traffic load imbalance. The RBS acting as source cell in the transfer calculates the load balancing amount, thus the number of UEs to be relocated, for such transfer. For calculating the load balancing amount, the RBS assesses the traffic load in its own cell and exchanges load information according to the set of established neighbor cell relations between the cells on different carrier frequencies. The RBS uses the load information to determine how much traffic, or how many UEs that should be relocated in order to reach load balance between neighbor overlapping cells.
5. When the RBS of the source cell has determined a load balancing amount towards a particular target cell, the RBS of the source cell selects one or more UEs reporting the particular target cell as the best neighbor cell on that carrier frequency to be part of the traffic load transfer. Only for UEs that are within coverage of the target load balancing frequency carrier, an IEF handover is performed.
The source cell RBS initiates IEF handover of the selected UE from the source cell to the target cell. The selection and IEF handover of the UE may continue until either the determined load balancing amount is reached, or a reassessment of the traffic load balance between the two cells is performed.
It is regarded that the load balancing mechanism, described above, focuses on the traffic volume presented to each cell. The fruitful purpose of load balancing, should be to ensure that the individual UE service performance is optimized on the same time as the total system performance, i.e. by not wasting any system resources.
Therefor the selection of the UEs to be relocated to the neighboring cells is regarded a problem as only a reliable selection results in a successful load-balancing wherein the system as a whole performs better than before the load balancing action, and the relocated individual UEs perceive an equal or better service.
In heterogeneous network deployments, where the RBS sites providing overlapping cells on different carrier frequencies are not co-located, there may be large differences in the received radio link quality for the individual UEs, depending on whether the UE is connected via a close-by RBS site on one frequency or a more remote RBS site on another frequency.
In an IEF load balancing example, the source RBS in an E-UTRAN network, typically requires a minimum received Reference Symbol Received Power (RSRP) for the UE from the target cell, before a UE is selected for handover in order to even out an unbalanced load between the two cells. However, if the UE receives good radio link quality in the source cell, a minimum RSRP in the target cell does not guarantee even a matching radio link quality in the target cell. If a UE “accidently” is relocated from a cell where it receives good radio link quality into a cell where it receives poor radio link quality it may consume a lot of radio resource from the network, just in order to overcome the worse radio link quality in the target cell, thereby worsening the overall capacity of the target cell and providing a worse performance for the relocated UE.
Hence, when selecting a UE for relocation for load balancing reasons, it is regarded a problem to select the particular UE in a way taking the expected radio link quality in both the source and the intended target cell into account.
The RBS in an E-UTRAN, has typically a good knowledge about the radio link quality the UE receives in the current serving source cell. This information can be obtained from the radio link adaptation performed for the UE in the source cell.
Prediction of the radio link quality and the performance the UE perceives in an intended target cell, when based on the RSRP and Reference Symbol Received Quality (RSRQ) values, measured from target cell signaling by the UE connected to the source cell are insufficient, in particular when load balancing occurs in heterogeneous network where cell patterns may differ substantially or where the conditions to the particular UE are quite different.
In addition to the radio link quality in source and target cells, the selection of the UE for relocation is made more complex by e.g. the RF bandwidths of target cells, carrier aggregation capabilities, MIMO RI received from the UE, etc. as the RF bandwidth and MIMO configuration of an E-UTRAN cell may vary within a rather wide range.